Amphibians
Gray Treefrog
Hyla versicolor · also called gray tree frog, eastern gray treefrog, common gray treefrog
The gray treefrog is a small, warty, colour-shifting frog that most people hear long before they see it — a rich trill from the trees around a pond on a summer night. It also does two things that sound impossible: it freezes solid every winter and survives, and it carries twice the normal number of chromosomes.
Overview
Hyla versicolor lives across eastern North America, in woodland near ponds and swamps. It is 3–5 cm long, with big sticky toe pads and skin that ranges from grey and lichen-mottled to bright green depending on temperature and surroundings. A hidden orange-yellow flash on the thighs shows when it jumps.
Frozen and revived
In autumn its liver pumps out glycerol and glucose as antifreeze, and the frog then lets roughly 40% of its body water turn to ice. Its heart stops. In spring it thaws from the inside, the heart restarts within hours, and it carries on.
A doubled genome
The gray treefrog is tetraploid — four chromosome sets instead of two — the result of ancient hybridisation. Its identical-looking twin, Cope’s gray treefrog, is diploid and gives a faster trill; the two are separated most reliably by call.
Status
The gray treefrog is Least Concern and stable, common in woods, gardens, and farm ponds across its range.
Gray Treefrog: common questions
How does the gray treefrog survive being frozen?
As the temperature drops, the frog's liver rapidly converts stored glycogen into large amounts of glycerol and glucose, which flood the blood and tissues and act as antifreeze. Ice still forms, but it is confined to the spaces between cells and outside the organs, and the sugars and glycerol inside the cells stop them from freezing and rupturing. In deep cold the frog is stiff, its heart is stopped, and it is not breathing. When spring warmth returns the ice melts from the inside out, the heart restarts within hours, and the frog resumes normal life. It can repeat this through the winter.
Why does a gray treefrog turn green?
The colour is controlled by pigment cells in the skin that expand and contract, shifting the frog between grey, brown, green, and pale tones over roughly half an hour. The change tracks temperature, light level, activity, and to some degree the background. A frog sitting on green leaves in warm weather often goes bright green; one pressed against grey bark on a cool day is grey and lichen-patterned. The species name versicolor means changeable in colour, and the two closely related gray treefrogs are among the more variable frogs in North America.
What is the difference between the gray treefrog and Cope's gray treefrog?
Almost nothing you can see. Hyla versicolor and Hyla chrysoscelis are so alike in size, colour, warty skin, and the orange thigh flash that they cannot be told apart reliably by appearance, and their ranges overlap widely. The real differences are internal and acoustic. Cope's gray treefrog is diploid, with the normal two chromosome sets, while the gray treefrog is tetraploid with four. Cope's also gives a faster, harsher, higher-pitched trill. Where both occur, the surest field identification is by listening to calling males on a warm night.
Are gray treefrogs good pets?
They are a common and hardy choice in the amphibian hobby. They are small, do not need a large enclosure, tolerate a range of conditions, and their colour changes and evening trilling make them interesting to keep. They need a planted vertical tank with branches, a shallow water dish, moderate humidity, and a supply of small insects, and like all amphibians they should not be handled often because their skin absorbs oils and salts. Captive-bred animals are preferable to wild-caught ones, though the species is abundant.